The present invention is concerned with making piston rings for internal combustion engines, and particularly with oil control rings and a method of making the same. More specifically, the present invention is concerned with a rolled steel oil control piston ring for internal combustion engines, i.e., with a method of making such rolled steel rings. However, the invention is not limited thereto, and concerns piston rings in general, and a method of making them.
Oil control rings for reciprocating piston internal combustion engines comprise split, generally circular rings which may be U-shaped in cross section. The rings are adapted to be received in a circumferential groove formed in the piston, with the free ends of the legs of the U (the sidewalls of the ring) projecting inwardly of the ring towards the bottom of the groove, and the connecting central portion of the U (the peripheral wall of the ring) facing outwardly of the groove, towards the cylinder wall. The outwardly facing peripheral wall has scraper rims formed thereon and the ring is thrust outwardly by an expander spring so that the rims engage the cylinder walls. The expander spring usually is disposed in the piston groove between the piston ring and the bottom of the groove, and engages the inside of the piston ring to force it outwardly.
Such oil control rings having a pair of flat-faced scraper rims to engage the cylinder walls, also have vents between the rims of the ring to admit the passage of debris therethrough, as understood by those skilled in the art.
For example, U.S. Pat. No. 3,378,268 assigned to the assignee of the present application, shows in FIG. 1 thereof an oil control ring assembly comprising a one-piece cast iron ring and an expander spring engaged therewith. The expander spring is between the bottom of the piston groove and the oil control ring and serves to expand the ring outwardly into contact with the cylinder wall of the engine. The cast-iron ring has rims formed in the peripheral wall thereof. The rims have flat scraping faces which contact the cylinder wall. The desired configuration of the ring is obtained by providing a casting of suitable shape which is thereafter machined to finished dimensions. Generally, castings are employed in the piston ring field, although it is conceivable that a ring of the desired configuration may be manufactured by a forging operation.
However, the use of cast iron oil control rings has some disadvantageous limitations. The degree of hardness which can be attained in a cast iron ring is limited by the ability to economically machine high hardness parts. If heat treatment to harden previously machined parts is used, part distortion of the small section rings is a problem. Further, cast iron rings are somewhat prone to damage upon installation within the groove of a piston. In addition, the expander spring normally employed in conjunction with the oil control ring is usually made of steel and, being harder than the cast iron ring, tends to wear those portions of the ring which it contacts to expand the ring.
The foregoing limitations can be avoided by forming the oil control ring from a hard metal such as steel. The manufacture of oil control rings by rolling from metal strip stock is also suggested in U.S. Pat. No. 3,378,268. For example, in FIGS. 5 to 7, a rolled oil ring formed from strip metal is shown. It will be appreciated that rolling the ring to proper profile from strip metal and thereafter coiling and cutting rings from the coiled, profiled strip is an efficient and much less expensive method of manufacture than the use of machined castings or of forgings.
However, certain difficulties are involved in rolling the desired profile from steel strip. One difficulty is that the required dimensional accuracy of the ring can not be easily maintained in the rolling operation. The tolerances required in the finished dimensions of piston rings and in particular of oil control rings are very small. For example, the flat face of the scraping rings must be very precisely aligned in flat scraping contact with the cylinder wall. This means that with the rings properly positioned in the groove, the flat faces of the rims must lie in a common plane which defines a cylinder closely conforming to the engine cylinder walls, to provide effective oil scraping thereof.
Obviously, much if not all of the cost and manufacturing advantage to be gained by rolling the profiled rings from steel strip will be lost if it is necessary to thereafter carry out a machining operation to assure satisfactorily close dimensional tolerances of the ring.
Further, prior methods of rolling the profiled ring from steel strips did not admit of providing the scraping rims with the desired flat face, but rather provided a rounded, sloping face to the rim as shown, for example, in FIGS. 5, 6 and 7 of the above mentioned U.S. Pat. No. 3,378,268.
The aforesaid patent also illustrates a prior art attempt to correct for large dimensional variations of rolled as opposed to machined cast rings. This attempt is to make the rolled ring from stock which is thin and flexible enough so that dimensional variations could be overcome by deformation of the rings against the cylinder wall surfaces by the expansion force of the expansion spring. (See FIG. 1 of 3,378,268.) Rings rolled from such relatively thin stock however, are inferior to rings of more rigid stock in terms of ring positional stability and oil scraping characteristics.
It is accordingly an object of the present invention to overcome these and other shortcomings of the prior art.
It is an object of the present invention to provide a method of making a rolled metal piston ring, and in particular a rolled metal oil control piston ring, which provides rings of closely controlled dimensional variations. It is another object of the invention to provide such a rolled ring which is of rigid construction, being made from relatively heavy stock to impart desired rigidity and improved scraping action to the ring, as compared to the structure of U.S. Pat. No. 3,378,268.
It is another object of the present invention to provide a method of rolling metal strip to produce from generally flat stock strip, such as a steel strip, a U-shaped cross section strip having scraper rims, preferably flat-faced rims, thereon and coiling and cutting the strip to form piston rings, and in particular, oil control piston rings therefrom.
In accordance with the present invention there is provided a piston ring comprising a rolled metal, preferably steel, ring of generally U-shaped cross section having at least one scraping rim formed in the peripheral wall thereof by a pleat folded from the wall and at least partially closed on itself. In a preferred embodiment, the rims are flat-faced. There are two such rims provided, and they extend circumferentially around the peripheral wall of the ring, which also contains vent holes in the ring, between the scraper rims. The U-shape cross section is provided by a centrally disposed peripheral wall from which opposed sidewalls project inwardly of the ring.
The method of the invention is carried out by introducing elongated metal strip of selected dimensions sequentially into a series of sets of profiled form rolls which define, between rolls of a set, metal work spaces. The strip may be considered to be comprised of a center portion bounded by two longitudinally extending edge portions. The metal, e.g., steel strip is passed sequentially into the series of the profiled rollers and through the metal work spaces defined therebetween. These serve to bend the strip into a transverse profiled cross section by confining both the edge portions of the strip by contacting them on both sides between opposed form rolls and deflecting both edge portions of the strip transversely about a longitudinal axis thereof to impart a generally U-shaped profile to the strip, while initially permitting displacement of the central portion of the strip within a freeboard space between form rolls provided for that purpose, in order to accomodate variations of the strip dimensions without cold-flow or other deformation of the edge portions. Additionally, the central portion is folded longitudinally to provide at least one (preferably, two) longitudinally extending pleats in the central portion of the strip. The pleats are at least partially closed on themselves to form layered rims projecting from the central portion of the strip.
After the initial forming of the U-shaped cross section profile, the strip is passed through an extruder-like work space between opposed roll forms in which it is subjected to sufficient pressure to impose an extrusion effect upon the metal by which the cross sectional area of the strip is reduced slightly to form the final cross sectional dimensions thereof.
In accordance with one aspect of the invention, the pleats are formed so that the finished rims have flat scraping faces.
The completed profiled strip is punched to form vent holes therein, and coiled into a helical coil to give a permanent coiled set to the strip. The coils may be severed to any desired coil length for storage or further processing. The scraper rims or the outer face of the ring including the scraper rims, may be provided with a wear resistant coating such as a chromium plated coating or a molybdenum coating. The coiled strip is then cut longitudinally so that a plurality of generally circular, gapped annular rings is formed therefrom.